There has recently been used a cell structure having a catalyst carried thereon in order to remove nitrogen oxides, sulfur oxides, sulfur chloride, hydrogen chloride, hydrocarbon, carbon monoxide, and the like, contained in an exhaust gas to cope with an automobile exhaust gas regulation which becomes more tighten with the years. The cell structure purifies automobile exhaust gas by adsorbing and decomposing the above harmful substances contained in an automobile exhaust gas with a catalyst carried on the partition walls. In addition, a cell structure is also used as a filter for trapping particulate matter discharged from a diesel engine.
Since a catalyst used in the above cell structure having catalyst carried thereon generally has high catalytic activity in a high temperature range, the catalytic activity is low from the engine start-up of an automobile till temperature of the cell structure rises, and insufficiently purified exhaust gas is discharged. Therefore, it is required to make the operating of a cell structure as short as possible.
As one of the method, there is a method in which heat capacity is reduced to raise temperature of the cell structure in a short time from the engine start-up of an automobile. It is necessary to lighten (lower of bulk density) the cell structure without changing the geometric surface area of the cell structure to reduce heat capacity of the cell structure. Therefore, there is a method in which thickness of cell partition walls is thinned or a method in which porosity is increased. However, the thinning of cell partition walls and the lowering of bulk density by increasing porosity causes deterioration in mechanical strength.
In addition, a cell structure for purifying exhaust gas discharged from a large-sized vehicle such as a truck at a high flow rate requires a large capacity, and a cell structure having a large sectional area is required in order to reduce a pressure loss. However, a cell structure having a large sectional area has a problem of easy deformation because the cell partition walls in the outer peripheral portion of the cell structure cannot bear its own weight during extrusion forming (see Patent Document 1).
In order to inhibit the mechanical strength from being deteriorated, there has been proposed a method in which, after the cell structure is formed and fired, the partition wall deforming region of the outer peripheral portion is worked and removed, and a shell layer constituting an outer surface is formed by filling the depressed groove of the outer peripheral portion by coating the outer peripheral portion with ceramic cement, and a cell structure provided with the shell layer (see Patent Document 2). However, though mechanical strength of the cell structure can be improved in this methods a heat capacity of the cell structure is increased, a temperature rise rate of the cell structure upon engine start-up is lowered, and it is difficult for the catalyst activity of the loaded catalyst to rise in a short time. Further, there is a problem of a temperature difference between the central portion and the outer peripheral portion of the cell structure. Such a temperature difference is not preferable because catalytic activity and regeneration of the filter become uneven when the cell structure is used as a catalytic carrier and a filter. In addition, it causes crack generation, which is not preferable.
In addition, there has been proposed a method in which a coating layer is provided on the outer peripheral surface of the honeycomb structure in which the partition walls and the outer peripheral wall are unitarily extruded, to improve accuracy in an outer diameter of the honeycomb structure (see Patent Document 3). This proposal discloses a method for inhibiting a honeycomb structure from breakage upon canning, which is caused by deterioration in mechanical strength of the honeycomb structure due to thinning of the partition walls, by reducing canning bearing by optimizing a clearance range upon canning by improving accuracy in an outer diameter of the honeycomb structure. However, though canning resistance of the honeycomb structure can be improved by such a means in which a coating layer is provided on the outer peripheral surface of the honeycomb structure having unitarily produced partition walls and the outer peripheral wall, there arise problems that heat capacity of the outer wall portion increases and that the heat inside the honeycomb structure transfers to the outer wall side, which are the same as the aforementioned honeycomb structure in which the outer periphery is coated with ceramic cement.
Patent Document 1: JP-A-3-275309
Patent Document 2: JP-A-3-269388
Patent Document 3: JP-U-63-144836